THE TRANSPORTATION OF LIQUIDS 115 pump is recognized by its spiral casing and tangential discharge nozzle, the turbine pump by its concentric casing and radial discharge nozzle. Velocity Diagram.—The water at the point of exit from the impeller has two velocities, viz.: The relative velocity va, tangential to the tip of the vane, and the peripheral velocity ua, tangential to the periphery of the impeller. These two velocities combine into the absolute velocity, which is indicated by the line marked wa in Fig. 9. The liquid leaves the impeller and enters the volute casing in the direction of this line wa. Here it encounters liquid flowing around the impeller in a more tangential direction and the results are large hydraulic losses, due to shocks, eddies and cross currents. These losses are more or less completely avoided in the turbine pump, hence its greater efficiency and the possibility of using higher velocities. The practical limiting velocities and heads in a volute pump with one impeller are ua = 120 ft. per second with a corresponding head of about 220 ft., while with a well-designed turbine pump ua may be = 140 ft. per second with a corresponding head of about 300 ft. FIG. 9.—Centrifugal pump velocity diagram. Higher velocities than the above are not practicable as the losses increase a the square of the velocity, and besides the high velocity may result in pitting and excessive wear of the impeller. The top of the vanes at the point of exit may be curved backward or forward, or may be radial. The first is used almost exclusively at the present date. Forward curved vanes are only used when it is desired to impart to the water a high velocity rather than pressure. In a similar way the "entrance diagram" can be shown which gives the relation of the various velocities to each other at the point of entrance to the impeller. The entrance diagram shown in Fig. 9 is the ideal entrance diagram which may differ considerably from the actual entrance diagram. In the former the water is assumed to enter the impeller in a radial direction, w8 representing the velocity of approach which is maintained as we also in a radial direction. The radial component wr is then also = we.